With recent growing demand for flat-screen TVs, display technologies of various styles, including those making use of liquid crystal, plasma, organic electroluminescence and field emission, have been developed. For any of these displays of different styles, a transparent electrode is an essential technology. Besides televisions, a transparent electrode is indispensable for touch panels, mobile phones, electronic paper, various solar batteries and various diming electroluminescent elements.
A conventional transparent electrode having been most widely used is an ITO transparent electrode manufactured by forming an indium-tin composite oxide (ITO) film by vacuum deposition or sputtering on a transparent substrate composed of glass, a transparent plastic film or the like. However, it has been desired to disuse indium because indium is a rare metal and its price has been soaring. There also has been desired a production technology of roll-to-roll using a flexible substrate (flexible transparent substrate), such as a resin substrate, for larger screens of displays and for improvement in productivity.
In recent years, there has been developed a transparent conductive film (refer to Patent Documents 1 and 2, for example) composed of: a thin metal wire formed in a pattern; and a conductive polymer and the like stacked thereon, and having both current uniformity in surface and high conductivity so as to be suitable for products which are demanded to have a large area and low resistance. This structure, however, necessitates smoothening asperity of the thin metal wire using a conductive polymer or the like, the asperity being causative of leakage in an organic electronic device, and therefore it is essential to thicken the conductive polymer. A conductive polymer, however, shows absorption in the visible light region. Therefore, if the conductive polymer is thickened, transparency of the transparent electrode significantly decreases.
As a method to achieve both conductivity and transparency, there have been disclosed: a technology of stacking a conductive polymer on a thin-wire structure portion (refer to Patent Document 3, for example); a technology of applying a conductive polymer and a binder resin which is uniformly dispersible in an aqueous solvent onto a conductive fiber (refer to Patent Document 4, for example); and a technology of stacking a conductive polymer and a binder on a conductive layer (refer to Patent Document 5, for example). However, even these technologies cannot achieve sufficient sheet resistivity and transmittance and accordingly have a problem in achievement of both conductivity and transparency.
By the way, when mass production of transparent electrodes is carried out by roll-to-roll using a flexible transparent substrate, such as a resin substrate, as a substrate of each transparent electrode, it is preferable to adopt “inkjet printing” as a method for forming a layer containing a conductive polymer in terms of a patterning property of the conductive layer, productivity and the like.
Meanwhile, in any of the technologies of Patent Documents 1 to 5, no consideration is given to problems of inkjet printing, such as clogging of a head, insufficient wettability of an ink to the substrate and insufficient dispersion stability of an ink over time. Further, for example, in the technology of Patent Document 4, the conductive polymer layer is formed by spin coating. Still further, in the technology of Patent Document 5, although drying at high temperature for a long time is necessary to sufficiently advance a polymer crosslinking reaction, this increases the process load and also disallows use of a resin substrate and a polymer each having a low glass transition temperature as materials for the transparent substrate and the conductive polymer layer, respectively, which is unsuitable for mass production of transparent electrodes by roll-to-roll.
Further, as described, for example, in Patent Documents 4 and 5, there has been developed, as a specific composition (application liquid) for forming a transparent electrode (conductive polymer layer), the composition containing a water-dispersible conductive polymer, such as 3,4-polyethylene dioxythiophene polysulfonate (PEDOT/PSS), and a binder resin in order to achieve both conductivity and transmittance. In particular, a hydrophilic binder resin has been investigated as a binder resin in terms of compatibility with a water-dispersible conductive polymer.